Re: GCC strcmp optimizations causing valgrind uninitialized conditional jumps

On Tue, Jul 03, 2018 at 11:59:14AM -0700, William Kennington wrote:

Is there a bug tracking the issue?

https://bugs.kde.org/show_bug.cgi?id=386945

Also, unless your malloc is
guaranteed to be zeroing out the data or have a strcmp that is writing
doubleworld aligned data to the string, the strcmp implementation is
branching based on data existing after the null terminating character
that may be uninitialized. Both sides of the branch do the right thing
though, and locate the null terminator, throwing away the calculations
done on the uninitialized data.

Yes, there is one branch that depends in part on irrelevant data, but
that is handled immediately afterwards.

-fno-builtin-strcmp or -mstring-compare-inline-limit=0 do work fine
but we don't control the binaries we are linking against in all cases
and are seeing the issue pop up there.

Yeah, nasty.  I don't know what to do then (other than fix valgrind, which
isn't so easy either though!)


Segher